1. Field of the Invention
This invention relates to a generator system for internal combustion engines which converts rotational energy into electrical energy and, more particularly, to a generator system for internal combustion engines which can produce electric power efficiently at all times irrespective of the engine speed even for use in an internal combustion engine operated at variable speeds such as a vehicle engine.
2. Description of the Related Art
A generator system for vehicles or ships includes an alternator (AC generator; ACG) with its rotating shaft coupled through an alternator belt to a crank shaft of an engine; a rectifier for converting AC power into DC power, the AC power generated by the alternator according to the engine speed; and a regulator for controlling the voltage level of the DC power according to the voltage at a battery.
FIG. 3 is a schematic diagram showing a structure of a conventional alternator 50, in which a DC field coil 53 is wound around a rotor 52 integrally formed with the rotating shaft and a three-phase coil 55 is wound around a stator 54. In the condition that the DC field coil 53 is excited by supplying direct current from the battery (not shown), the rotor 52 is rotated to create an alternative magnetic field. Consequently, the three-phase coil 55 of the stator 54 generates AC power with a frequency corresponding to the rotation speed of the rotor 52. Such a conventional alternator is an AC generator using a synchronous motor. In some cases, the rotor 52 may be provided with a permanent magnet instead of the DC field coil 53.
In recent vehicles, power consumption has increased because of current tendencies to electrify or electronically control vehicle parts including the engine and to widely use audio and navigation systems. This requires the alternator to increase the capacity of power generation. The electric power generated by the alternator is reduced as its rotation speed drops. It is therefore necessary to increase the ratio of the alternator speed to the engine speed so that sufficient generated power can be obtained even at low engine speeds such as 1000 r.p.m or less. In many cases, the pulley ratio is set twice or more for that purpose.
On the other hand, high velocity revolution and low idling of the engine is accelerated as the engine output and efficiency increase. For this reason, the ratio of the minimum and maximum engine speeds is becoming larger. When the pulley ratio is set higher for the electric power required at low engine speeds, the alternator speed tends to exceed the maximum rate determined from mechanical restrictions in high velocity revolution. This also causes increased fan noise at high revolution of the engine and hence degradation of the engine. Further in the generator system for vehicles, since the crank shaft of the engine and the rotating shaft of the alternator are coupled by the alternator belt at all times, rotation losses in the alternator (e.g., belt driving torque and/or cooling fan resistance) occurs even when the alternator is not required to generate electric power due to fully charged battery. This causes lowered engine efficiency or increased fuel cost.
As previously discussed, the conventional generator system for internal combustion engines operated at variable speeds such as vehicle engines can not take a highly set ratio of the alternator speed to the engine speed from the point of view of the durability and the quality or performance of the alternator. This causes insufficient generated energy especially in a range of low velocity revolution of the engine.
In order to set the ratio of the alternator speed to the engine speed arbitrarily, for example, Japanese Patent Publication No. sho 62-33465 proposes a mechanism for mechanically changing the diameter of the alternator driving pulley. However, such a mechanism for mechanically changing the speed causes a complicated and large-scale structure.